I've not been in FB but I believe they have recommended responses, a button that you can hit for a quick response, with less typing. Maybe that is the new etiquette, send this mindless response, which indicates interest. It's one or two clicks. You can respond to a bunch of ads and keep fishing until you get the deal you want.
Plus... I'm guessing it's like CL, where one can put up a post, sell it, and then decide to not take down the listing. And ignore any responses (maybe it goes to spam, maybe it was a burner account).
Seems like if I were to call someone up about an item (remember those days?), the first question would be, do you still have said item? The internet is faster than print copy but can have the same issues. Thus is this not basically the same response?